1. Field of the Invention
The invention generally relates to magnetic transducers, and more particularly to magnetic sensors used with microelectromechanical system (MEMS)-type components.
2. Description of the Related Art
Conventionally, most magnetic sensors used to measure electrical current flow have a limited dynamic range. If they are very sensitive, they will saturate and become inoperative in larger magnetic fields. As such, there is a need to increase this dynamic range. The measurement of electrical currents may be affected by 1/f noise and by signals from other sources. For example, it is difficult to measure small 60 Hz currents because of the presence of 60 Hz background noise. Thus, there is a need to mitigate the effect of 1/f noise and other sources of magnetic fields.
Interest is increasing in the development of miniature sensors for sensing magnetic fields in extraterrestrial, industrial, biomedical, oceanographic, and environmental applications. The trend in magnetic sensor design and development is constantly toward smaller size, lower power consumption, and lower cost for similar or improved performance.
There are several types of magnetometers, which are magnetic sensors with external instrumentation, currently used. The least expensive and least sensitive devices have a resolution of approximately 10−1 Oersted (Oe)/Hz1/2 and typically are Hall effect devices. These devices operate by sensing a voltage change across a conductor or semiconductor placed in a magnetic field. Such devices do not lend themselves for applications requiring greater sensitivity, such as required in brain scan devices and magnetic anomaly detection devices.
Flux gate magnetometers are more sensitive, having a resolution of approximately 10−6 Oe/Hz1/2. Flux gate magnetometers use a magnetic core surrounded by an electromagnetic coil, and are difficult to microfabricate. Additionally, flux gate magnetometers require a relatively large amount of power and accordingly do not lend themselves to a low-cost, compact, portable design.
The most sensitive magnetometers called SQUIDS (superconducting quantum interference detectors) have a resolution of approximately 10−10 Oe/Hz1/2. However, because SQUIDS include a superconducting element, these apparatus must include cooling means at liquid gas temperatures, making them extremely bulky and expensive to operate. Also, their relatively large size limits their utility because the active superconducting element cannot be placed directly adjacent to the source of the magnetic field. As such, it is common in magnetic sensors to place the sense element between two stationary flux concentrators to enhance the field.
Accordingly, there is a need for small, inexpensive, low power magnetometers, which have an even greater sensitivity than the conventional devices, and which are useful for a variety of magnetometer applications at low frequencies. Magnetoresistive sensors are suited for low-cost, medium-sensitivity applications. For example, using spin-dependent tunneling magnetoresistive sensors, one can observe 38% changes in the resistivity in fields of a few Oe. See, e.g., D. Song, J. Nowak & M. Covington, J. Appl. Phys, 87, 5197 (2000), the complete disclosure of which is herein incorporated by reference.
Furthermore, magnetic sensors used to detect objects that move slowly typically possess considerably low frequency 1/f-type noise, which is an unwanted condition. Generally, there is a tendency for such devices, which have higher sensitivity, to also exhibit higher 1/f-type noise. This type of noise generally occurs when using magnetoresistive-type magnetic sensors. See, e.g., van de Veerdonk et al. J. Appl. Phys. 82, 6152 (1997), the complete disclosure of which is herein incorporated by reference.
Another problem with many magnetic sensors is that the change in signal due to the magnetic field is small compared with the background signal that will be referred to as the DC offset. For example, in spin valve giant magnetoresistor sensors, the change is about 5–10%. For anisotropic magnetoresistance sensors the change is even smaller. Extracting the signal from the DC offset requires using carefully constructed bridges and other techniques.
A well known way of increasing the sensed magnetic field by a magnetic sensor is by use of a flux concentrator, which can enhance a sensed magnetic field by as much as a factor of 50. See e.g., N. Smith et al., IEEE Trans. Magn. 33, p. 3358 (1997), the complete disclosure of which is herein incorporated by reference. An example of such a device is taught in Popovic et al., U.S. Pat. No. 5,942,895, issued Aug. 24, 1999, entitled “Magnetic field sensor and current and/or energy sensor,” the complete disclosure of which is herein incorporated by reference, which teaches the use of Hall effect sensors with flux concentrator components.
A magnetic sensor (magnetometer) which addresses 1/f-type noise is taught in Hoenig, U.S. Pat. No. 4,864,237, issued Sep. 5, 1989, the complete disclosure of which is herein incorporated by reference. Hoenig teaches an apparatus for measuring magnetic fields, which change only at extremely low frequency. Hoenig uses a SQUID magnetometer, which includes a superconducting flux transformer that inductively couples a detected signal into a DC SQUID sensor.
The magnetometer of Hoenig may include a device for modulating the detected signal in a frequency range characteristic of low-noise operation of the SQUID. The modulation frequencies are generally above 1 Hz and optionally even above 1 Khz. However, the limitations of this device, and others like it, include the need for a cryogenic operation, which inherently does not lend to relatively low cost, low power use.
In U.S. Pat. No. 6,501,268 issued to Edelstein et al. on Dec. 31, 2002, the complete disclosure of which is herein incorporated by reference, it is indicated that MEMS flux concentrators use torsional motion of MEMS flaps around torsional suspensions driven by the electrostatic field applied between two parallel capacitance plates.
The above-referenced techniques were satisfactory for the purposes for which they were intended. However, there continues to remain a need for small, low-cost, low-power-consuming magnetic sensors having enhanced sensitivities capable of meeting the varied applications listed above for detecting low frequency signals and minimizing 1/f-type noise. Also, there remains a need for an improved MEMS device with a greater dynamic range of motion to allow for larger modulation of an electrical signal.